The present invention generally relates to a double polarized light beam spectrophotometer of a light-source modulation type and in particular to a double polarized light beam spectrophotometer of the light-source modulation type which is advantageously suited for use in Zeeman atomic absorption type photometry.
In the spectrophotometer, it is known to produce a modulated light beam produced by a light source by correspondingly modulating electric power supplied to the light source for excitation thereof. The use of the modulated light thus produced is certainly effective for eliminating noise components generated in a specimen chamber or the like.
In the accompanying drawings, FIG. 1 is a view for graphically illustrating comparatively a relationship between a modulating frequency and a noise frequency. The optical noise 2 produced in the specimen chamber or the like is predominant in a relatively low frequency region. In contrast, the modulating frequency (f.sub.O) of the light source is selected at a relatively high value, as indicated by a line 1. Accordingly, even when the noise frequency and the modulating frequency coexist optically, the modulating frequency component 1 of the light source can be easily discriminated from the noise 2 generated in the specimen chamber or the like after both the frequencies have been converted into electric signals. For example, a major part of the noise components 2 generated in the specimen chamber can be eliminated by using a band-pass filter having a center frequency corresponding to the modulating frequency f.sub.O or by using a high-pass filter for passing therethrough the frequency components higher than the frequency f.sub.O, whereby a satisfactory S/N ratio can be realized. However, it has been found that the double polarized light beam spectrophotometer of the light-source modulation type with which the present invention is concerned can not assure operation with a satisfactorily high accuracy merely by resorting to the use of the filters of the type mentioned above.
For example, in the double polarized light beam spectrophotometer of the light-source modulation type in which the Zeeman effect is made use of, the modulated light radiated by the light source undergoes Zeeman atomic absorption by specimen vapor and is subsequently subjected to selection of wavelength through a monochromator. Thereafter, the light of the selected wavelength is spatially separated into a polarized light component termed the sample light which is in parallel with the magnetic field and a polarized light component referred to as the reference light which is perpendicular to the magnetic field. The sample light beam and the reference light beam pass alternately through a rotary chopper to be received by a common photomultiplier tube. The signal resulting from the photoelectric conversion is of such waveform as shown in FIG. 2 in which time is taken along the abscissa with the current intensity being taken along the ordinate. The modulation of the light source is illustrated by vertical lines, wherein the maximum light emission is indicated by the vertical lines among which the minimum light levels intervene. High and low levels in magnitude of the arrayed vertical lines are explained by the fact that the sample light indicated by 4 and the reference light indicated by 3 and 5 are alternately changed over in a double beam optical system. It is assumed that the switching frequency at which the sample and the reference light are changed over is 50 Hz (i.e. the number of the switching times is 100 per second) and that the modulating frequency of the light source is 1483. 33 Hz. That is, the ratio of the switching frequency of the chopper to the modulating frequency is at the ratio of 3 to 89. So long as this ratio is accurately maintained, there exists deviation of 1/3 of the modulation period of the light source between the preceding reference light 3 and the succeeding reference light 5, as a result of which the phase of the reference light coincides with the phase of the modulated light every third period, giving rise to generation of a beat signal having a frequency of 16.7 Hz which corresponds to 1/3 of the switching frequency of 50 Hz.
Besides, since the ratio 3:89 of the two frequencies mentioned above will not be perfectly stabilized but will fluctuate slightly, a beat signal of another frequency may often be produced. For example, when the ratio of 3 to 89.1 is slightly varied to 3 to 39.1 which is equivalent to 10:297, there is produced a beat signal of 5 Hz which is 1/10 of the switching frequency. In this way, when the modulation of the light source and the switching or changing-over between the sample light and the reference light are controlled independent of each other, there is inevitably produced a beat signal having a frequency which is necessarily subjected to variations. Thus, great difficulty is encountered in eliminating this type of beat signal.
Next, examination will be made as to the magnitude of the amplitude at which the beat components make an appearance in the detected output signal. Referring to FIGS. 3A and 3B which correspond to the view shown in FIG. 2 and enlarged along the abscissa, there is illustrated how much of the light quantity can be received by a photomultiplier tube during an interval of 10 m sec (because of 100 switchings per second) of the sample light or reference light. It is assumed that the light source is sinusoidally modulated and that the frequency component of 14.833 . . . of the light source is received, although the overall light amount as received depends on the position of the modulated light source. FIG. 3A shows the case where the light quantity or amount received by the photomultiplier tube is at a maximum, while FIG. 3B shows the case where the received light amount is at a minimum. In these figures, components 6 and 7 of 14 periods are shown discriminatively from the components 8 and 9 of 0.833 period. The component of 0.833 period illustrated in FIG. 3A covers an area of 0.993 while the periodic component 9 of 0.833 period shown in FIG. 3B covers an area of 0.674.
However, since the area for one period is standardized to 1 (unit), the ratio of area occupied by the two components mentioned above is 14.674 to 14.993. Accordingly, when the light absorption in the case illustrated in FIG. 3A is assumed to be zero, the light absorption in the case illustrated in FIG. 3B corresponds to 0.009.
In the foregoing, the frequency of the beat generated in the hitherto known spectrophotometer has been examined in conjunction with FIG. 2, while examination has been made by referring to FIGS. 3A and 3B as to the possibility of the beat having an amplitude corresponding to the light absorption of 0.009. In this way, generation of the beat is inevitable in the prior art spectrophotometry, involving a large burden on the signal processing for reducing such beat signals while notwithstanding rendering it very difficult to attain a satisfactory accuracy or precession in the displayed value of measurement, to a serious disadvantage.
In case a graphite atomizer is used for atomization or vaporization of the specimen, a commercial frequency current of 400 A at maximum is supplied to the atomizer. As a consequence, the atom vapor generated in the atomizer undergoes fluctuation at the commercial line frequency, bringing about a variation in frequency in the Zeeman atomic absorption. Since this frequency variation will correspond to the switching frequency of the chopper described hereinbefore, beat signals will also be produced due to the phenomena discussed above in conjunction with FIGS. 2, 3A and 3B.